The present invention relates to an image adjusting method by an electrophotographic system and an image forming apparatus and a memory product for recording a computer program to implement the image forming apparatus, more particularly, to an image adjusting method and an image forming apparatus for adjusting color misregistration of a multicolor image occurring at the time of forming the multicolor image by superposing a plurality of color component images, and to a memory product for recording a computer program to implement the image forming apparatus.
In image forming apparatuses such as a digital color copier and a digital color printer, an input image data is decomposed into color components and processed, after which the images of the color components are superposed to form a multicolor image. In forming a multicolor image, a failure in accurately superposing the images of the color components causes a color misregistration of the multicolor image formed, thus deteriorating image quality. Especially in an image forming apparatus having an image forming unit for each color component to improve the speed of forming a multicolor image, an image of each color component is formed in each image forming unit, and the images of the respective color components are sequentially superposed thereby to form a multicolor image. This image forming apparatus poses the serious problem that the transfer position of the image of each color component is liable to deviate which causes a color misregistration of the multicolor image.
In view of this, an image forming apparatus has been proposed in which in order to superpose the images of the color components accurately, the color matching adjustment is conducted for correcting the color misregistration of the multicolor image thereby to form a satisfactory multicolor image free of a color misregistration. In the color matching adjustment, the displacement of an image forming position of a given color component with respect to an image forming position of a reference color component is detected by an optical sensor. Based on the result of this detection, an adjusting value for the image forming position is calculated, and in accordance with this adjusting value, the timing of forming an image of each color component is adjusted in such a manner as to assure coincidence of the transfer positions of the images of the respective color components. In order to calculate the adjusting value, the image of each color component is transferred at the same timing and the distance between the transfer positions of the respective color components is detected or the concentration of the multicolor image with the color components superposed is measured.
In an image forming apparatus for detecting the distance between the transfer positions of the images of the respective color components and correcting the color misregistration based on the amount of displacement of the detected transfer positions, for example, the distance between the image formed of a reference color component and the image formed of other color components is detected by a detector, and based on the detected distance, the amount of displacement of the transfer position of each color component is determined thereby to correct the color misregistration (Japanese Patent Application Laid-Open No. 10-213940(1998)).
Also, an image forming apparatus has been proposed, in which the concentration of a multicolor image with images of the color components superposed is measured, and the color misregistration is corrected in such a manner that the concentration measurement represents a concentration with the images of the color components accurately superposed (Japanese Patent Application Laid-Open No. 2000-81744). With this image forming apparatus, in order to improve the correction accuracy, the image of each color component of the same shape is repeatedly formed. As an image of the same shape, a plurality of line segment images (line images) are formed and the concentration of a multicolor line image is detected by a detector thereby to determine the superposed state of the line images of the color components. A state in which the concentration of the multicolor line image detected by the detector is in a predetermined concentration range is regarded as a state in which the line images of the color components are accurately superposed one on another. The color misregistration is corrected and the color matching adjustment is conducted in such a manner as to form an image in this superposed state.
However, in the image forming apparatus for determining the transfer position of each image using a detector for detecting the transfer position of the image of each color component, the detection of a minor displacement of the transfer position requires a detector of high detection accuracy.
The conventional image forming apparatus for color matching adjustment using the line images described above, on the other hand, requires the operation to determine a correction value at which a reference color component image (reference image) and a color component image to be adjusted (adjustment image) are superposed completely one on the other, while shifting the adjusting value for each line over the entire area of color matching adjustment. This poses the problem that the concentration is required to be detected for correcting the color misregistration over the entire area in the range where the color matching adjustment is possible, thereby consuming a long time for color matching adjustment, and in the case where the adjustment time is to be reduced, a sufficiently wide area for color matching adjustment cannot be secured.
Further, the method in which the superposition of the reference image and the adjustment image is detected by the image concentration in the particular image forming area poses the problem that a plurality of reference images and adjustment images are required to be formed to assure stable detection, thereby increasing the amount of the developer consumed while at the same time lengthening the time required for color matching adjustment.